A quantum cascade laser (QCL) can be used with an external cavity to detect and/or measure the concentration of one or more specific gases in a gas sample. To perform such detection, a flat mirror can be positioned in the external cavity to reflect an incident beam generated by the QCL back upon itself in a direction aligned with (i.e., parallel to) the direction of the incident beam. In addition, a grating can be used to generate beams of different wavelengths for propagation through the gas sample. Signals resulting from such propagation can be used to measure the concentration of one or more gases in the gas sample.
One disadvantage associated with this arrangement is that it is difficult to align a flat mirror perpendicularly relative to a QCL beam in an external cavity because such a beam is often an invisible, infrared beam. Another disadvantage is that, if a movable flat mirror is used to alter the external cavity length dynamically (e.g., to provide mode hop free wavelength scanning), the motion mechanism used to move the flat mirror needs to maintain the mirror perpendicularly to the incident QCL beam while in motion, which can be a difficult and costly endeavor. Yet another disadvantage is that frequent adjustment of the flat mirror and/or the grating may be needed because beams of different wavelengths originate from different parts of a QCL layered structure. Therefore, a beam originating from the QCL, after propagating through a collimating lens, can be aimed in slightly different directions in the external cavity depending on its wavelength. If no adjustment is made to the flat mirror or the grating, the output beam can fail to lase, change amplitude, mode hop to an unpredictable wavelength, and/or fail to be correctly correlated to the grating tilt angle.